This chapter explores the critical role of completion equipment and wellhead systems in establishing and maintaining well integrity throughout the life of the well. It begins with an overview of completion objectives and system classification (e.g., open hole vs. cased hole, single vs. multizone, and intelligent completions), highlighting how equipment selection directly influences barrier reliability and intervention capability. The chapter details the design, functionality, and installation practices of key completion components including packers, tubing, subsurface safety valves (SSSVs), flow control devices, and production liners. Emphasis is placed on the mechanical integrity, pressure ratings, corrosion resistance, and seal effectiveness of each component under expected downhole conditions. Wellhead systems and Christmas trees are also thoroughly examined in terms of structural design, load-bearing capacity, pressure containment, and barrier redundancy. API standards (e.g., API 6A, 17D) and testing requirements are integrated to ensure compliance and operational safety. The chapter includes failure case analyses related to completion leakage, packer slippage, tubing collapse, and wellhead seal erosion, supported by root-cause diagnostic techniques and integrity management workflows. Additional sections address the importance of elastomer compatibility, thermal cycling effects, and wellhead fatigue under cyclic loads. By consolidating engineering principles with practical field insights, the chapter equips engineers with a robust foundation for selecting, installing, and maintaining completion and wellhead systems that uphold barrier integrity under dynamic production, injection, and stimulation conditions.